Is the "specified line" the one in blue, or the one with the # already in front of it? Does the two sets of lines illustrate input and output or do they all belong to the same file?
– Kusalananda♦Jan 12 '17 at 7:47

The "specified line" the one with the # already in front of it
– user210015Jan 12 '17 at 8:00

How we're supposed to find the "specified line" ? Is it by line number ? Is it by specific text ? Is it by specific search pattern ? Please clarify your question
– Sergiy KolodyazhnyyJan 12 '17 at 8:02

This can be shortened with awk '$0=="Hellow2"{print "#"$0;next};1' input.txt. The pattern before each code block {} can be treated as if statement already. 1 Just tells awk to print the line, that's all
– Sergiy KolodyazhnyyJan 12 '17 at 8:07

@Serg Thanks for your info, you are totally right, but, i wanted it to be clear
– Wissam RoujoulahJan 12 '17 at 8:09

but it do not save it to the file.. how can I save it?
– user210015Jan 12 '17 at 8:11